Mike, that was very informative thank you.I had no idea. My piece of plywood is not marked interior or exterior but when I went to the company website it only described the red oak under interior plywoods it offered.

Hardwood plywoods (like oak) are not normally available in Exterior grade (especially off the shelf). Even if you purchase exterior grade plywood make certain it is well sealed especially the edges (fill the voids of you can). Just because it is exterior grade which basically means exterior grade glue, it will still delaminate if moisture gets in between the veneers.

Typically hardwood plywoods like oak is going to be for interior use. There are marine grade hardwood plywoods, but you'd have to go looking for them and they're definitely not going to be at your box big home improvement store.